The document discusses the 90/10 principle created by Stephen Covey, which states that 10% of life is made up of external events outside of one's control, while 90% is determined by an individual's reaction to what happens. It provides an example of how reacting negatively to a small mishap like spilled coffee can negatively impact one's entire day, while responding positively can prevent issues from arising. The principle teaches that people have control over how they react but not what initially happens, and choosing to react in a calm, rational manner allows one to be much happier and avoid stress.